Method of manufacturing an optical data storage medium, optical data storage medium and apparatus for performing said method

ABSTRACT

A method of manufacturing an optical data storage medium, comprising at least one substrate ( 11 ) and a plurality of layers deposited on the substrate ( 11 ) is described. The medium includes at least one of a transparent spacer layer and transparent cover layer ( 12 ). The layer ( 12 ) is provided by applying a liquid onto the rotating substrate ( 11 ) and rotating the substrate ( 11 ) further in order to spread out the liquid into a layer substantially uniformly between an inner radius ri and an outer radius ro, and solidifying the liquid layer ( 12 ) by means of exposure to UV radiation. After applying the liquid onto the rotating substrate the liquid layer ( 12 ) is heated by heating means ( 14 ) in such a way that the temperature rise of the liquid layer ( 12 ) at ri has a value dTri, while the temperature rise of the liquid layer ( 12 ) between ri and ro gradually increases, and the temperature rise of the liquid layer ( 12 ) at ro has a value dTro&gt;dTri. In this way the spacer layer or cover layer has a variation in thickness smaller than +/−1 μm, measured over the information storage area. Further a medium manufactured using said method and an apparatus for performing the method are described.

The invention relates to a method of manufacturing an optical datastorage medium, comprising at least one substrate and a plurality oflayers deposited on the substrate, including at least one of atransparent spacer layer and transparent cover layer, which layer isprovided by applying a liquid onto the rotating substrate and rotatingthe substrate further in order to spread out the liquid into a layersubstantially uniformly between an inner radius r_(i) and an outerradius r_(o), and solidifying the liquid layer by means of exposure toUV radiation.

The invention further relates to an optical data storage mediummanufactured using said method.

The invention further relates to an apparatus for performing saidmethod.

An embodiment of such a method is known from European patent applicationEP-A-1047055. In particular the application of a light transmissiveadhesive layer in order to bond cover layers or other layers to eachother, to the surface of a substrate and/or to one or more informationstorage layers is described.

There is a constant drive for obtaining optical storage media suitablefor recording and reproducing, which have a storage capacity of severalGigabytes (GB) or larger.

This requirement is met by some Digital Video Disk or sometimes alsoDigital Versatile Disk formats (DVD). DVD formats can be divided intoDVD-ROM that is exclusively for reproduction, DVD-RAM, DVD-RW andDVD+RW, which are also usable for rewritable data storage, and DVD-R,which is recordable once. Presently the DVD formats comprise disks withcapacities of 4.7 GB, 8.5 GB, 9.4 GB and 17 GB.

The 8.5 GB and, in particular, the 9.4 GB (DVD-9) and 17 GB (DVD-18)formats exhibit more complicated constructions and usually comprisemultiple information storage layers. The 4.7 GB single layer re-writableDVD format is easy to handle comparable, for example, to a conventionalcompact disk (CD) but offers an insufficient storage capacity for videorecording purposes.

A high storage capacity format that recently has been suggested isDigital Video Recordable disk (DVR). Two formats are currently beingdeveloped: DVR-red and DVR-blue, the latter also called Blu-Ray Disk(BD), where red and blue refer to the used radiation beam wavelength forrecording and reading. This disk overcomes the capacity problem and, inits simplest form, has a single storage layer format which is suitablefor high density digital video recording and storage having a capacityup to about 22 GB in the DVR-blue format.

The DVR disk generally comprises a disk-shaped substrate exhibiting onone or both surfaces an information storage layer. The DVR disk furthercomprises one or more radiation beam transmissive layers. These layersare transmissive to the radiation beam that is used to read from orwrite into the disk. For example a transmissive cover layer, which isapplied on the information storage layer. Generally, for high-densitydisks, lenses with high numerical aperture (NA), e.g. higher than 0.60,are used for focusing such a radiation beam with a relatively lowwavelength. For systems with NA's above 0.60 it becomes increasinglydifficult to apply substrate incident recording with substratethicknesses in the 0.6-1.2 mm range due to decreasing tolerances on e.g.thickness variations and disk tilt. For this reason, when using disksthat are recorded and read out with a high NA, focusing onto a recordinglayer of a first recording stack, is performed from the side oppositefrom the substrate. Because the first recording layer has to beprotected from the environment at least one relatively thin radiationbeam transmissive cover layer, e.g. thinner than 0.5 mm, is used throughwhich the radiation beam is focused. Clearly the need for the substrateto be radiation beam transmissive no longer exists and other substratematerials, e.g. metals or alloys thereof, may be used.

In case second or further recording stacks are present, a radiation beamtransmissive spacer layer is required between the recording stacks. Thesecond and further recording stacks must be at least partiallytransparent to the radiation beam wavelength in order to making writingin and reading from the recording layer of the first recording stackpossible. The thickness of such spacer layers typically is from theorder of tens of micrometers. The radiation beam transmissive layer orlayers which are present between the radiation beam source and therecording stack that is most remote from the substrate are normallycalled cover layers. When prefabricated sheets are used as transmissivelayers extra transmissive adhesive layers are required in order to bondcover layers to each other.

In the DVR disk the variation or unevenness of the thickness of theradiation beam transmissive layers over the radial extension of the diskhas to be controlled very carefully in order to minimize the variationin the optical path length for the impinging radiation. Especially theoptical quality of the radiation beam at the focal point in the DVR-blueversion, which uses a radiation beam with a wavelength substantiallyequal to 405 nm and an NA substantially equal to 0.85, is relativelysensitive to variations in the thickness of the transmissive layers. Thetotal layer thickness has an optimal value in order to obtain minimumoptical spherical aberration of the focused radiation beam on, e.g., thefirst information recording layer. A deviation, e.g. +/−5 μm, from thisoptimal thickness already introduces a considerable amount of this kindof aberration. Because of this small range it is important that theaverage thickness of the transmissive layers is equal to or close to itsoptimal thickness in order to make optimal use of the tolerances of thesystem and to have a high yield in manufacturing the medium. Assumingthat a thickness error is Gaussian distributed around the nominalsetting of the thickness, it is clear that the number of manufactureddisks which do not comply with the above specification is minimal whenthe target setting of the nominal thickness during manufacture issubstantially equal to the optimal thickness of the cover layer as inthe specification of the DVR disk. The nominal thickness of a singlelayer cover of the DVR disk is 100 μm when the refractive index of thecover layer is n=1.6. The nominal thickness of the cover layer has to beadjusted when using a different refractive index. Since a change inoptimal thickness can exceed more than one micron, it is clear from thepoint of view of yield that even this small change has to be taken intoaccount.

As described earlier, multi-stack disks, e.g. dual-stack, are used toincrease the storage capacity of disks. Those disks require atransmissive spacer layer in between the recording stacks. In the caseof the dual recording layer DVR disk the sum of the thickness of thespacer layer and the cover layer is chosen to be 100 μm, e.g. a 25 μmspacer layer and a 75 μm cover layer. From EP-A-1047055 it is known touse a polymer layer such as, for example, a polycarbonate (PC) sheet aslight-transmissive cover or spacer layer and adhere such layer to theinformation storage layer by means of a thin, spin-coated layer of a UVcurable liquid resin or a pressure sensitive adhesive (PSA). Because thedisk now is built up of more than one radiation beam transmissive layerit becomes even more difficult to manufacture the disk which varieswithin the above specified range. Hence for such a disk it is even moreimportant to set the nominal thicknesses substantially equal to theoptimal nominal thicknesses of the cover and spacer layers of the disk.

In order not to depend on measures for compensation of sphericalaberration in an optical drive when playing or recording e.g. a BD disk,the thickness variation of the cover layer of a single recording stackdisk should be smaller than +/−2 μm. For e.g. a dual recording stack BDdisk that variation relates to the spacer layer and cover layerthickness and should be smaller than +/−1 μm for each layer separately.As said before, this puts even more stringent requirements in thetolerance of each separate layer.

A technique currently used by some manufacturers for producing a spacerlayer is DVD-bonding. Firstly, spincoating provides an auxiliarysubstrate or “stamper”, e.g. a PC substrate with guide grooves, with athin layer non-adhesive to the stamper, which is subsequently cured orsolidified with ultraviolet (UV) radiation. Then, this auxiliarysubstrate or “stamper” is glued to a DVD substrate with known DVDbonding techniques, in which technique the liquid glue is spincoatedwhile present between the two substrates and subsequently cured byexposure to UV radiation. The circumferential variation of the totalthickness of the cured non-adhesive layer and glue layer cannot becontrolled well and the necessary tolerance, e.g. +/−1 μm, is not met.Furthermore, the spin coating application of the non-adhesive layerintroduces a so-called edge bead effect at the edge of the disk. This isa peripheral zone of e.g. a few mm with relatively largely increasedlayer thickness because of surface tension effects at the edge of thedisk. An increase of the layer thickness of larger than 5 μm may occurin this zone. Subsequently the stamper is separated from thenon-adhesive layer that remains glued to the second substrate. Furtherprocess steps follow to finalize the DVD medium, e.g. the application offurther recording stacks and a cover layer.

Another method comprises the application of a “PSA-like” material whichis UV-cured after being brought in contact with the first DVD substrateunder vacuum. This material is usually supplied as a sheet of foil.Thickness variations achieved with this material may be below +/−2 μm.However due to the high material costs such a process is relativelyexpensive compared to a spincoating process.

When using a known spin coating process the following problems areencountered. Because the substrate usually contains a center hole theliquid to be solidified is dosed in the form of a circular bead aroundthe center hole. This usually results in a liquid layer which afterrotation of the substrate yields a liquid layer which shows a radiallyincreasing layer thickness of 15-30% from inner to outer diameter of theliquid layer. The edge bead at the peripheral zone of the substrate mayresult in an extra layer thickness increase of more than 5 μm, e.g. fromradius 55 to 58 mm when using a 120 mm diameter circular substrate.Usually these edge phenomena are not uniformly present around theperiphery resulting in additional circumferential variations at theouter periphery area of the substrate. When using spin coating with e.g.the DVD-bonding technique glue which is expelled from between the DVDsubstrate and the stamper may accumulate at the periphery and leave aresidue at the stamper or a burr at the DVD substrate after separatingthese two. This poses a problem for reuse of the stamper and the burr atthe edge of DVD substrate may cause problems in subsequent process stepsof manufacture of the optical data storage medium e.g. application of atransparent cover layer.

It is an object of the invention to provide a method of the kinddescribed in the opening paragraph, for manufacturing an optical datastorage medium with a spacer layer or cover layer which has a variationin thickness smaller than +/−1 μm, measured over the information storagearea.

It is another object of the invention to provide an optical data storagemedium with a spacer layer made according to the method of the inventioncontaining embossed information.

It is a further object of the invention to provide an apparatus forperforming the said method.

The first object is achieved with a method according to the openingparagraph which is characterized in that:

-   -   after applying the liquid onto the rotating substrate, the        liquid layer is heated by heating means in such a way that,    -   the temperature rise of the liquid layer at r_(i) has a value        δT_(ri) while,    -   the temperature rise of the liquid layer between r_(i) and r_(o)        gradually increases,    -   the temperature rise of the liquid layer at r_(o) has a value        δT_(ro)>δT_(ri).

When using this method the viscosity of the liquid layer is decreasedaccording to its temperature rise. This viscosity decrease influencesthe fluid flow physics of the spin coating process in such a way thatthe radial liquid layer thickness profile after spincoating issubstantially uniform. Fine tuning for better uniformity may be achievedby e.g. changing the rotation frequency of the substrate or by changingthe rotation period, although these changes are of secondary effect. Infact the shape of the temperature rise profile is the main factordetermining the uniformity of the desired final radial thicknessprofile. The desired uniformity of the profile is that the radialthickness distribution of the liquid layer after complete substantialsolidification does not have a variation of more than +/−1 μm.

Preferably the temperature rise between r₁ and r₀ has a radialtemperature profile with a shape substantially resembling the shape of aradial thickness profile resulting when δT_(ro) and δT_(ri) would bezero. When using this profile a liquid layer thickness variation of evenless than +/−0.5 μm may be achieved.

In an embodiment the heating means comprise an infra red heating deviceprojecting IR radiation onto the substrate in an area with a radiuslarger than r_(i) for causing a desired radial temperature profile inthe liquid layer. This heating method has the advantage of beingrelatively easy to implement. Alternatively the heating means comprise aheated chuck on which the substrate is mounted during rotation, saidchuck having a heated surface for causing a desired radial temperatureprofile in the liquid layer or the heating means comprise a directedflow of heated gas emanating from a nozzle.

It is advantageous when a few mm wide outer peripheral zone of thesubstrate is shielded by a mask in order to prevent exposure of theliquid layer in this zone to UV radiation. After the exposure of theliquid in the exposed portion, the substrate is rotated at a rotationfrequency sufficiently high to substantially remove the non exposedliquid in the outer peripheral zone from the substrate. This has theadvantage that a possible edge bead (see 12 b in FIG. 1) in the outerperipheral zone is removed and that no residues of liquid are left atthe outer periphery of either the substrate or a stamper which is usedwith e.g. the DVD bonding technique as described earlier in which caseUV curable glue, which is expelled from between the DVD substrate andthe stamper and has accumulated at the periphery and leave a residue atthe stamper or a burr at the DVD substrate after separating these two,is removed by this process step. In this way a stamper may be used againmore easily. Compare the so-called DVD-18 technique, which is used toproduce double sided dual layer DVD read only disks where information istransferred by embossing via a stamper substrate, but which techniquerequires the removal of excess glue in order to enable a good separationof the stamper substrate and the DVD substrate.

It is especially advantageous when the exposure takes place in anatmosphere containing oxygen and at an exposure intensity leaving a fewμm top portion of the liquid layer unsolidified by means of oxygeninhibition. In such a way the top portion of the liquid layer is leftsubstantially unsolidified. This enables the embossing of information,e.g. pregrooves or pits or the inverse, in the top portion of the liquidlayer. Small relative variations in the thickness of the top portion,e.g. 0.2 μm, may still occur but are negligible compared to the totalthickness of the liquid layer after solidification.

The second object is achieved with an optical data storage medium asdescribed in the second paragraph which is characterized in that astamper is pressed into the unsolidified top portion of the liquid layerof a spacer layer manufactured using the method of the invention.Subsequently the top portion is solidified by exposure to radiation. Thestamper is separated from the top portion of the completely solidifiedliquid layer. Further layers, e.g. recording stacks and a cover layer,may be provided for finalization of the optical data storage medium. Byleaving the top portion of the liquid layer substantially unsolidifiedinformation may be embossed without noticably disturbing the totalthickness of the spacer.

In a favorable embodiment the stamper is transparent to UV radiation andthe top portion is solidified by UV radiation which is projected throughthe transparent stamper. A transparent stamper has the advantage ofenabling a more direct exposure of the top portion of the liquid layerand that a substrate may be used which is not transparent to UVradiation.

The third object is achieved with an apparatus comprising:

-   -   means for receiving a substrate and a plurality of layers        deposited on the substrate,    -   means for rotating the substrate,    -   means for providing at least one of a transparent spacer layer        and transparent cover layer, by applying a liquid onto the        rotating substrate and rotating the substrate further in order        to spread out the liquid into a layer substantially uniformly        between an inner radius r_(i) and an outer radius r_(o), and    -   means for heating the liquid layer after applying the liquid        onto the rotating substrate in such a way that,    -   the temperature rise of the liquid layer at r_(i) has a value        δT_(ri) while,    -   the temperature rise of the liquid layer between r_(i) and r_(o)        gradually increases,    -   the temperature rise of the liquid layer at r_(o) has a value        δT_(ro)>δT_(ri).    -   means for solidifying the liquid layer by exposure to UV        radiation directly after the heating step.

In an embodiment the means for heating comprise an infrared heatingdevice projecting IR radiation onto the substrate in an area with aradius larger than r_(i) for causing a desired radial temperatureprofile in the liquid layer.

In another embodiment the means for heating comprise a heated chuck onwhich the substrate is mounted during rotation, said chuck having aheated surface for causing a desired radial temperature profile in theliquid layer.

In another embodiment the means for heating comprise a directed flow ofheated gas emanating from a nozzle for causing a desired radialtemperature profile in the liquid layer.

Preferably the apparatus has a mask for shielding a few mm wide outerperipheral zone of the substrate in order to prevent exposure of theliquid layer in this zone to UV radiation.

The method of manufacturing the optical data storage medium and theoptical data storage medium according to the invention will beelucidated in greater detail with reference to the accompanyingdrawings, in which

FIG. 1 shows a schematic cross-sectional view of a setup to perform anembodiment of the method according to the invention; The dimensions arenot drawn to scale;

FIG. 2 shows a device in which an optical data storage mediummanufactured with the method according to the invention is present andembossed with a transparent stamper.

FIG. 3 shows a thickness (t) profile of a liquid layer after UV curing,applied without heating step as a function of radius (r);

FIG. 4 shows a radial thickness profile of a liquid layer after UVcuring, applied using a heating step according to the invention;

FIG. 5 shows irradiance distributions of the IR lamp used in oneembodiment of the invention;

FIG. 6 shows a radial temperature profile of the liquid layer measured afew seconds after the heating step by means of an IR camera

In FIG. 1 a setup for performing an embodiment of the method ofmanufacturing an optical data storage medium is shown. The mediumcomprises a substrate 11 with a plurality of layers, e.g. a recordingstack, which is not drawn. A transparent spacer layer 12 is provided byapplying in 6 seconds about 2 grams of a liquid 12 onto the rotatingsubstrate 11 and rotating the substrate 11 further in order to spreadout the liquid 12 substantially uniformly between an inner radius ofr_(i)=23 mm and an outer radius of r_(o)=57.5 mm. The rotation frequencyof the substrate during application is ⅔ Hz and is subsequently rampedup to 50 Hz in about 3 sec and then left another 5 seconds on 50 Hz. Theliquid is a UV curable glue provided by Eques having a viscosity of 1000mPas. When, during the spreading of the liquid, the liquid has reachedthe outer edge the liquid layer 12 is heated by heating means duringabout 4 seconds, i.e. in such a way that,

-   -   the temperature rise of the liquid layer 12 at r_(i) has a value        δT_(ri) while,    -   the temperature rise of the liquid layer 12 between r_(i) and        r_(o) gradually increases,    -   the temperature rise of the liquid layer 12 at r_(o) has a value        δT_(ro)>δT_(ri).

More preferably the temperature rise between r_(i) and r_(o) has aradial temperature profile with a shape substantially resembling theshape of a radial thickness profile resulting when δT_(ro) and δT_(ri)would be zero, i.e. without heating. This radial thickness profile isshown in FIG. 3.

The heating means comprise an infra red heating device 14 projecting IRradiation onto the substrate 11 in an area with a radius larger thanr_(i) for causing the desired radial temperature profile in the liquidlayer 12. The IR lamp 14 is a 500 W source with a heating length of 272mm having an IR-3 reflector. Irradiance profiles are shown in FIG. 5.The optical axis of the reflector has an angle of about 45 degrees withthe substrate surface as drawn in FIG. 1. The distance between edge ofthe reflector and the substrate is kept small, e.g. 2 mm. The edge ispositioned at a radius of about 24 mm, i.e. relatively close to theinner radius r_(i).

A radial thickness profile is achieved with a thickness of 25 μm. Thevariation is not more than +/−0.5 μm. In order to minimize changes inthe thickness profile the rotation frequency is ramped down from 50 Hzto 13 Hz in about 2 seconds and left 10 seconds at this rotation speedin order to allow time for removing the IR heating device and placing aUV radiation source.

Subsequent pre-solidifying of the liquid 12 is performed by means ofexposure to UV radiation, e.g. a high power UV source 15, e.g. PhilipsHP-A 400 W, with a special reflector at a height of 10 cm above theliquid 12 surface. UV radiation source 15 gives a substantially uniformradiation output. The UV exposure for pre-solidifying at the position ofthe liquid layer 12 of the substrate takes 2 seconds with an intensityof 50 mW/cm². The UV exposure takes place in an atmosphere containingoxygen, i.e. air, and at an exposure intensity leaving a few pm topportion of the liquid layer 12 unsolidified by means of oxygeninhibition. This top layer may be required for subsequent process steps,e.g. the embossing of information in the top surface of the liquid layer12. A few mm wide outer peripheral zone of the substrate 11 is shieldedby a mask 16 in order to prevent exposure of the liquid layer in thiszone to UV radiation. After the UV exposure of the liquid 12 in theexposed portion, the substrate 11 is rotated at a rotation frequencysufficiently high, i.e. 65 Hz, to substantially remove the non exposedliquid 12 b in the shape of an edge bead in the outer peripheral zonefrom the substrate 11. Note that in the drawing the layer thickness ofthe liquid layer 12 is drawn showing the situation before starting theheating method according to the invention. It must be noted that thementioned spin coating rotation speeds and times may be adapted and thatat higher rotation speeds the cycle time may be reduced substantially.In a more automated process using a higher intensity of both the IR lampand the UV lamp even a further reduction of cycle time may be achieved.The IR and UV radiation sources may be positioned automatically, whichfurther reduces the cycle time. The method may be fine tuned to liquidswith different properties, e.g. viscosity. E.g. for DVD bonding a glueis used, made by DIC type nr SD694, having a viscosity of 350 mPas. Thespin rotation speeds for this liquid must be adapted to e.g. 30 Hz and10 Hz instead of 50 Hz and 13 Hz.

In FIG. 2 a device 20 is shown to manufacture an optical data storagemedium, e.g. containing two or more recording stacks separated by spacerlayers. A stamper 23 transparent to UV radiation is pressed into theunsolidified top portion of the precured liquid layer 22 manufacturedaccording to the method of the invention. Subsequently the top portionis solidified by exposure to UV radiation which is projected through thetransparent stamper 23. The transparent stamper 23 is separated from thetop portion of the completely solidified liquid layer 22. Further layersare provided separately for finalization of the optical data storagemedium. Note that a non-transparent stamper, e.g. Ni, may be used incase exposure of the liquid layer 22 is performed from a side other thanthe side where the stamper is present.

The functioning of device 20 for manufacture of a medium will now beexplained in more detail. The substrate 21, provided with a liquid layer22 according to the method of the invention is positioned on top of aholder 29 in the bottom part 20 a of the device and at the same timeprecentered. The holder 29 is connected to the rest of the bottom part20 a of the device by means of a rubber membrane 28. At the beginning ofthe process, vacuum is present under the holder 29 of the bottom part 20a The top part 20 b of the device 20, holds a transparent stamper 23which is held in position against the top part 20 b by means of a vacuumoutlet 25. The centering pin 24 a around which the stamper 23 iscentered is tapered and will center the substrate 21 with precuredliquid layer 22 when parts 20 a and 20 b are brought together. Airbetween the substrate 21 and stamper 23 is pumped out through opening 26until a desired under-pressure level is achieved. Subsequently air islet into the bottom part 21 a through opening 27, which action pressesthe substrate 21 with layer 22 against the stamper 23. UV radiation issent through the transparent plates 24 and the transparent stamper 23 ina desired dose and cures the top portion of the layer 22. Air isreleased back into through 36 and after opening the device 20 thestamper may be separated from the substrate 21 with cured layer 22. Thetop surface of cured layer 22 now contains a negative copy of the reliefstructure of the stamper 23.

In FIG. 3 a radial (r) thickness (t) profile of the liquid layer,measured after UV curing, is shown achieved when δT_(ro) and δT_(ri)would be zero, i.e. when no heating step is performed. It can be seenthat a substantial increase in layer thickness is present radiallyoutward, which is far from the desired profile. However it was foundthat when the temperature rise between r_(i) and r_(o) has a radialtemperature profile with a shape substantially resembling the shape of athis radial thickness profile a very flat final thickness profile can beachieved as shown in FIG. 4. resulting when δT_(ro) and δT_(ri) would bezero. The achieved temperature profile may be adjusted by moving theposition of the heating means, e.g. the IR-lamp. A temperaturemeasurement of the surface of the liquid layer after the heating stephas been performed with an infrared camera as described with FIG. 6.

In FIG. 4 a radial thickness profile is shown achieved with the methodaccording to the invention.

In FIG. 5 the irradiance distribution of the IR-3 reflector used for oneembodiment of the invention is shown at different spacings (distances).

In FIG. 6 the measured radial temperature profile of the heated liquidlayer is shown as measured with an IR camera. This measurement can onlybe done after switching of the IR lamp. Hence the measurement isperformed a few seconds later and therefore the temperature profile isonly an indication of the real temperature profile. Note that thehorizontal axis (radius) is reversed.

It should be noted that the above-mentioned embodiment illustratesrather than limits the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. In the claims, any reference signsplaced between parentheses shall not be construed as limiting the claim.The word “comprising” does not exclude the presence of elements or stepsother than those listed in a claim. The word “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.The mere fact that certain measures are recited in mutually differentdependent claims does not indicate that a combination of these measurescannot be used to advantage.

According to the invention a method of manufacturing an optical datastorage medium, comprising at least one substrate and a plurality oflayers deposited on the substrate is described. The medium includes atleast one of a transparent spacer layer and transparent cover layer. Thelayer is provided by applying a liquid onto the rotating substrate androtating the substrate further in order to spread out the liquid into alayer substantially uniformly between an inner radius r_(i) and an outerradius r_(o), and solidifying the liquid layer by means of exposure toUV radiation. After applying the liquid onto the rotating substrate theliquid layer is heated by heating means in such a way that thetemperature rise of the liquid layer at r_(i) has a value δT_(ri), whilethe temperature rise of the liquid layer between r_(i) and r_(o)gradually increases, and the temperature rise of the liquid layer atr_(o) has a value δT_(ro)>δT_(ri). In this way the spacer layer or coverlayer has a variation in thickness smaller than +/−1 μm, measured overthe information storage area. Further a medium manufactured using saidmethod and an apparatus for performing the method are described.

1. A method of manufacturing an optical data storage medium to form atleast one substrate having a plurality of layers deposited on thesubstrate, including at least one of a transparent spacer layer and atransparent cover layer, the method comprising acts of: applying aliquid onto the rotating substrate by rotating the substrate further inorder to spread out the liquid into a layer substantially uniformlybetween an inner radius r_(i) and an outer radius r_(o), and exposingthe liquid layer to UV radiation to solidify the liquid layer; andheating the liquid layer by heating means in an area with a radiuslarger than the inner radius r_(i) in such a way that the temperaturerise of the liquid layer at r_(i) has a value δT_(ri) wherein while thetemperature rise of the liquid layer between r_(i) and r_(o) graduallyincreases, the temperature rise of the liquid layer at r_(o) has a valueδT_(ro)>δT_(ri), and wherein the temperature rise between r_(i) andr_(o) has a radial temperature profile with a shape substantiallyresembling the shape of a radial thickness profile resulting whenδT_(ro) and δT_(ri) are zero.
 2. The method as claimed in claim 1,wherein the heating act is performed by heating means having an infrared heating device for projecting IR radiation onto the substrate in anarea with a radius larger than r_(i) for causing a desired radialtemperature profile in the liquid layer.
 3. The method as claimed inclaim 2, wherein the heating means comprise a heated chuck on which thesubstrate is mounted during rotation, said chuck having a heated surfacefor causing a desired radial temperature profile in the liquid layer. 4.The method as claimed in claim 1, wherein the heating means comprise adirected flow of heated gas emanating from a nozzle for causing adesired radial temperature profile in the liquid layer.
 5. The method asclaimed in claim 1, wherein a few mm wide outer peripheral zone of thesubstrate is shielded by a mask in order to prevent exposure of theliquid layer in this zone to UV radiation.
 6. The method as claimed inclaim 5, wherein after the exposure of the liquid layer in the exposedportion, the substrate is rotated at a higher rotation frequency thanpreviously applied that is sufficiently high to substantially remove thenon exposed liquid in the outer peripheral zone from the substrate. 7.The method as claimed in claim 1, wherein the act of exposing isperformed in an atmosphere containing oxygen and at an exposureintensity, the act of exposing further comprises an act of inhibitingoxygen for leaving a top portion of the liquid layer unsolidified. 8.The method as claimed in claim 1, wherein a thickness of the liquidfollowing the heating of the liquid varies over an entire surface of theoptical data storage medium in a range of +/−1.0 μm.
 9. The method asclaimed in claim 1, wherein a thickness of the liquid following theheating of the liquid varies over an entire surface of the optical datastorage medium in a range of +/−0.5 μm.
 10. The method as claimed inclaim 1, wherein a thickness of the liquid following the heating of theliquid varies over an entire surface of the optical data storage mediumin a range of +/−0.2 μm.